The invention relates to a method and apparatus for reducing the data rate of a digital measuring signal produced in Fourier Spectroscopy. In this connection, in a double beam interferometer the moving mirror of which is displaced at a speed which is as constant as possible, the interferogram of an unknown measuring beam to be analysed is produced and is converted into a first electrical analog signal of high frequency.
The electrical signal produced in this manner corresponds to the Fourier transform of the energy spectrum, thus to the beam strength in dependence upon the wavelength of the interfering beam. The relationship between the optical frequency K of the spectrum and the electrical frequency F is given by: EQU F=v.multidot.k, (1)
in which v is the optical displacement speed of the moving mirror in the interferometer. From the above equation (1) it is apparent that with interferometers having a high displacement speed of the mirror a measuring signal of correspondingly high electrical frequency is obtained. This frequency typically lies in the range of 10 to 500 KHz. The interferogram signal is usually analysed with digital computers. For this purpose, the electrical measuring signal must first be digitalized. According to the sampling theorem of information theory, the analog signal must be sampled at a frequency which is at least double the signal frequency in order to enable a true reconstruction of the original interferogram or the spectrum. With the mentioned high frequencies of the measuring signal, this leads to very high data rates of the digitalized signal whose subsequent further processing and storage in the associated computer thus gives rise to problems.
The interferogram produced by the interferometer contains all information concerning the spectrum of the measuring beam to be analysed from frequency zero up to the upper optical boundary frequency. Generally, not all the frequency range of the spectrum is interesting for the analysis but only a particular spectral region characterized by the lower cut-off frequency F.sub.u and the upper cut-off frequency F.sub.o. The upper cut-off frequency F.sub.o of the spectral range of interest normally coincides with the upper optical boundary frequency of the interferogram. Even if only the upper spectral range of the interferogram is analyzed with the associated computer, for example by filtering out of the lower frequencies with an electrical high pass filter, the problem of high data rates of the digitalized measuring signal, which are difficult to handle, in substance remains.